The present invention relates to a technique concerned with nozzle touch in an injection molding machine in which two injection units are retained in parallel on one heating cylinder retention member.
A twin injection unit type injection molding machine in which two injection units are retained in parallel on one heating cylinder retention member can divide and supply raw resin of one and the same lot into the two injection units. Thus, the raw resin is injected concurrently from the two injection units so that the variation in quality between parts molded concurrently can be made as small as possible. For example, in production of optical disk substrates to be laminated to each other, two optical disk substrates molded concurrently and having small variation in optical performance therebetween can be laminated to each other. Thus, an optical disk superior in quality can be formed.
FIGS. 3 and 4 are diagrams showing the configuration of a twin injection unit type injection molding machine manufactured by way of trial by the present inventors before the present inventors conceived the present invention. FIG. 3 is a partially cutaway plan view of a main portion of the injection molding machine, in which a part of a heating cylinder retention member is omitted. FIG. 4 is a right side view of FIG. 3, depicting the whole outline of the heating cylinder retention member.
In FIGS. 3 and 4, the reference numeral 51 represents a fixed die plate; 52A and 52B, fixed-side molds mounted on the fixed die plate and having one and the same shape; 53, a heating cylinder retention member (head stock); 54A and 54B, heating cylinders having their base portions retained on the heating cylinder retention member 53, disposed in parallel with each other and having one and the same shape; 55A and 55B, nozzles attached to the front ends of the heating cylinders 54A and 54B respectively and having one and the same shape; 56A and 56B, nozzle touch rods disposed bilaterally symmetrically outside the two heating cylinders 54A and 54B respectively, having their one ends fixed to the fixed die plate 51, and having one and the same shape; 56A-1 and 56B-1, ball screw shaft portions of the nozzle touch rods 56A and 56B; 57A and 57B, nut pieces retained rotatably by the heating cylinder retention member 53 and screwed down to the ball screw shaft portions 56A-1 and 56B-1 respectively; 58A and 58B, driven pulleys fixedly attached to the nut pieces 57A and 57B respectively; 59, a servo motor provided for nozzle touch and mounted on the heating cylinder retention member 53; 60, a driving pulley fixedly attached to the output shaft of the servo motor 59; and 61, a timing belt hung among the three pulleys, that is, the driving pulley 60 and the driven pulleys 58A and 58B, and for transmitting the rotation of the servo motor 59 (driving pulley 60) to the driven pulleys 58A and 58B.
In the configuration shown in FIGS. 3 and 4, the rotation of the servo motor 59 for nozzle touch is transmitted to the driven pulleys 58A and 58B through the driving pulley 60 and the timing belt 61, so that the nut pieces 57A and 57B rotating together with the driven pulleys 58A and 58B rotate relatively to the ball screw shaft portions 56A-1 and 56B-1 respectively. Thus, the nut pieces 57A and 57B move linearly along the ball screw shaft portions 56A-1 and 56B-1 respectively so that the heating cylinder retention member 53 and hence the two injection units move linearly together with the nut pieces 57A and 57B.
Nozzle touch of the two injection units is performed as follows. That is, when the nozzles 55A and 55B of the two injection units are separated from the fixed-side molds 52A and 52B so as not to touch them respectively, the servo motor 59 is driven to rotate in a predetermined direction. As a result, due to the aforementioned transmission system, the heating cylinder retention member 53 (two injection units) is driven to advance toward the molds so that the front ends of the nozzles 55A and 55B of the two injection units are pressed onto the peripheries of resin injection holes of the fixed-side molds 52A and 52B respectively (FIG. 3 shows the nozzle touch state). In the nozzle touch state, the servo motor 59 keeps generating predetermined torque so as to retain a predetermined nozzle touch force.
However, in the twin injection unit type injection molding machine in which two injection units are retained in parallel on one heating cylinder retention member 53 as described above, the two injection units indeed have totally the same shape in design, but there is a dimensional error within a tolerance between the paired injection units. Due to the dimensional error, which is unavoidable in manufacturing, there occurs a difference of about 0.1–0.2 mm in whole length between the paired injection units.
When there is a difference in whole length between the two injection units in such a manner, for example, when the whole length of the upper injection unit is longer than the whole length of the lower injection unit in FIG. 3, the nozzle 55A of the upper injection unit touches the mold earlier. Once the nozzle 55A of the upper injection unit is pressed onto the mold with a predetermined nozzle touch force, the power of the servo motor 59 does not act to press the nozzle 55B of the lower injection unit further toward the mold though the nozzle 55B of the lower injection unit has not yet obtained a predetermined nozzle touch force. Thus, the nozzle 55B of the lower injection unit is short of the nozzle touch force. The arrows in FIG. 3 schematically show a difference in nozzle touch force. A relation in force size opposite to the arrowed relation will be established if the whole length of the lower injection unit is longer than the whole length of the upper injection unit. Such a situation is unavoidable due to the configuration in which the driven pulleys 58A and 58B of the two injection units are driven concurrently by the single servo motor 59 for nozzle touch. Thus, once one of the injection units is short of nozzle touch force as described above, there occurs a problem that resin leaks from the front end of the nozzle of the injection unit short of the nozzle touch force.